Removal Of Trichloroethylene In Aqueous Solution Using Coupled Adsorption-Catalytic Hydrodechlorination System

The issue of TCE contamination in groundwater has been widely concerned for many years.The effective control of TCE contamination has great significance to the safety of aquatic environment and the health of human beings.The present research intends to probe the possibility of application of montmorillonite supported palladium catalysts in practical remediation of groundwater contamination.To achieve this goal,we constructed coupled adsorption-catalytic hydrodechlorination system which was a process that mixed the catalysts and contaminant at first and then introduced formic acid or formate as hydrogen transfer into the solution for simulating the procedure of in situ groundwater contamination processing.On the basis of the positive result that TCE in the aqueous phase could be removed effectively by this system,we continued to study the effects of cations in the interlayer of catalysts supports,pH of solution,coexisting anions and organic substances on the removal of TCE.The results showed that montmorillonite supported Pd catalysts synthesized by ion exchange method remains the physical and chemical characterization of montmorillonite,and Pd particles as the shape of "platelet" lie in the interlayer of catalysts.The batch experiments based on the coupled adsorption-catalytic hydrodechlorination system exhibited that TCE in the aqueous solution(4 mg·L-1)could be degradated completely within 5 h when using HDTMA-Pd-MMT,with ethane as final product via direct hydrodechlorination approach.Moreover,the kinetic profile of TCE conversion was basically fitted zero-order kinetic model and the adsorption of HCOOH/HCOO-to active sites,catalytic composition of HCOOH/HCOO-as well as the production of active hydrogen([H])were the main limited step in the whole reaction system.And under the same reaction conditions without extra substances existing in the solution,Pd-MMT exchanged with different cations would not differ in the catalytic activity in the coupled adsorption-catalytic hydrodechlorination system.However,the catalytic hydrodechlorination of TCE with hydrogen as hydrogen donor was fitted by pseudo-first-order model and TCE could transform sequentially to ethane.Besides,catalysts exchanged with different cations presented difference in catalytic activity and the internal diffusion of TCE in the catalysts supports which is greatly influenced by hydration of cations and hydrophobicity of organic cation could account for this.In addition,chemical condition of reaction solution had influences to the coupled adsorption-catalytic hydrodechlorination system.With pH increasing,the transformation rate of TCE decreased,and TCE degrading would be suppressed completely at basic condition,which might attribute to the ratio of HCOOH/HCOOreducing with pH rising and HCOOH with the higher activity and the stronger adsorption on the Pd surfaces.As to catalytic hydrodechlorination,the higher pH helped to facilitate the removal of TCE because the basic condition could improve the dispersion stability of montmorillonite and weaken the internal diffusion resistance of TCE in the catalysts supports.Moreover,the study showed that both of coexisting chloride and humic acid were harmful to the coupled adsorption-catalytic hydrodechlorination system because of the catalytic poison effects,and Pd-MMT exchanged with different cations showed diversed inhibition behavior,for which the different diffusion capacity of outer substances to interlayer space of clay controlled by the varying d-spacing of montmorillonite could account.The layer distance of clay was mainly determined by not only the hydration capacity of cations but also the long alkyl chain of organic cation.Besides,organic cation also had stronger adsorption affinity to HA via hydrophobic partition which could protect Pd active sites from HA poisoning to some extent.Though much improvement and further study for application of the coupled adsorption-catalytic hydrodechlorination system in the practical in situ remediation of TCE groundwater contamination need to be done,the present research has proved this new system is successful to remove TCE in the aqueous solution.The idea that alternates low soluble and difficult-to-store hydrogen molecule with completely dissolved formic acid/formate as hydrogen donor as well as uses cheap and easy-to-obtain,natural and eco-friendly montmorllonite as supports of catalysts are able to achieve environmental safety and economy.In the practical contamination remediation,we could adjust and control the cations in the interlayer of catalysts by ion exchange to acquire particular characterization of catalysts for meeting the environmental needs.The results of this work could provide the theoretical guide to remediation of groundwater contaminated by chlorinated organic compounds and has great significance to environmental protection.